14.8 Translucent Sheeting

Translucent sheets should be fastened in the same way as metal cladding, except that extra or special fixings are required because plastic sheeting deflects more.

A different depth or screw gun torque setting may be required to avoid over tightening and subsequent distortion around the fixing. Because of their different rates of thermal expansion, plastic and metal sheet fixings should be fastened through oversized holes. Support for plastic sheeting is obtained from the metal sheeting for downward loads and span-breakers for uplift loads. See 14.8.4 Midspan Supports.

All plastic roof light sheets must be laid on top of the metal sheeting at both edges.

Side laps must be fixed at a maximum of 600 mm centres (fixing details in 14.8.1 Side-Lap Fixing) and are required to be sealed where the roof pitch is less than 10° and in high wind design load areas.

Oversized holes and load-spreading washers must be used when fixing roof lights to metal sheeting. All plastic roof lighting must have pre-drilled intermediate side lap fastenings with oversized holes on all spans larger than 1.2 m. See 14.8.1 Side-Lap Fixing

To avoid stress, roof lights should always be fixed starting from one end and fixing the intermediate purlins before doing the ends.

Deflection limits should be placed on the performance of plastic sheeting to ensure longevity and to avoid incremental failure due to flexing at the fixing points. This can be done by using mid-span supports. Plastic sheeting should not be used in applications where it approaches its deflection limits (50 mm).

Shallow symmetrical profiles of plastic roof lighting can be curved to suit draped curved metal cladding. However, the radius is determined by the profile, material type and thickness. See 15.1 Curved Roofs

When long lengths of plastic sheeting are not supported at mid-span, their longevity is reduced and incremental failure can be caused by the flexing at their fixing points.

The two main factors that limit spanning capability are deflection and pull-through over the fasteners.

As standard weight plastic roof lights have a low pull-over load and the critical condition is that imposed by wind suction; their performance is determined by the number of fasteners per square metre.

Load-spreading washers specifically made to fit the plastic sheeting must be used under all fasteners.

All fixings and washers must meet the same durability standard as the roof and wall cladding.

All plastic sheeting must have an enlarged fixing hole for the provision of thermal movement, predrilled to the following diameters for 12# or 14# self-drilling screws for both timber and steel purlins.

The fixing must be positioned in the centre of the oversize hole to ensure that the sheet has equal movement in all directions around the fixing.

For PVC and PolycarbonateSheets up to 4 mSheets up to 6 mSheets over 6 m

10mm12mmNR

The minimum fixing pattern for roof lighting cladding is determined by the span, the design load and the profile thickness, but fixings with load-spreading washers are recommended for all profiled ribs, except for spans less than 1 m.

Tray profiled GRP roof sheeting must be mechanically fixed to provide for the uplift load specific to the site, and must be placed on top of the adjacent metal sheeting.

In extreme environments where there are chemical fumes and the fixings are prone to chemical attack from the inside of a building, stainless steel fixings should be used.

When pan fixing is used, the fastener should be positioned within 25 mm of the rib.

14.8.1 Side-Lap Fixing

As there is no capillary bead formed on the side laps of plastic roof lighting, the fixing should be in the top and not the side of the rib. The overlap rib of the translucent sheeting should have a minimun clearance of 10 mm from the pan to avoid capillary action.

The side-laps of GRP sheeting and of the adjacent sheet must be fixed through the top of the rib with oversize holes in the GRP and load spreading and sealing washers. (see drawing 9.7.1.)

14.8.2 Side Lap Fixing Between Purlins

The side-lap fixing between purlins of GRP sheeting and the adjacent sheet must be fixed at maximum 600 mm centres through the top of the rib, with oversize holes in the GRP and load spreading and sealing washers.

When fixing GRP to GRP, a rubber-nut or grommet can be used and installed from the top side of the roof sheet using a 35 mm long, 4.5 mm diameter, aluminium or stainless steel gutter bolt and a 20 mm diameter metal and EPDM sealing washer.

Alternatively, if Bulb-Tite rivets are used a clearance hole is required, and care should be exercised when setting the rivet to avoid stress at the connection. This fixing is not suitable for use in high design wind load areas.

Underlay or foil must be terminated at the lap to provide continuity for any condensation that may occur.

When fixing tray decking profiles and using matching roof lighting sheets, there are several fixing options depending on the configuration of the profile. The independence of the metal and plastic sheeting should be maintained with all profiles to allow for their differing rates of expansion.

14.8.3 End Laps

The minimum end lap of GRP sheets with metal or GRP profile sheeting must be 200 mm. These laps must be sealed, and the bottom end of the overlapped sheet must be within 50 mm of the lower side of the purlin or saddle flashing.

Two lines of sealing beads or compressible strips must be placed, one within 15 mm from the bottom of the top sheet and the other 15 mm from the top of the bottom sheet.

Only compatible sealants must be used with plastic sheeting.

Silicone sealants should not be used with polycarbonate roof lighting.

The sealant materials for end laps of plastic sheeting should be neutral cure silicone, EPDM closed cell foam with self-adhesive on one face 25mm x 3 - 5mm thick, or polythene butyl tape.

14.8.4 Midspan Supports

Unless the thickness of the plastic sheeting is increased to equal the performance of the metal sheeting, mid-span supports or span breakers with a minimum thickness of 0.95 mm coated steel must be used as a mid-span support for all purlin spacings over 1.2 m.

Mid-span supports must not be used for more than one plastic sheet next to each other. Where two or more adjacent sheets are used together, the purlin spacings must be reduced to the maximum allowable for the profile, the thickness of the sheet and the design load.

A mid-span support is used at mid-span at spacings, dependent on the profile, to gain additional support from the metal cladding for the plastic sheeting.

Single sheets of plastic roof sheeting placed between adjacent metal sheets can only have the same purlin spacings as the metal roofing profile when a mid-span support is used, or the plastic sheeting thickness has been increased to provide the same performance characteristics as the metal sheeting. The exception to this condition applies only when the plastic sheeting has been tested for point loads as described in 17.7 Wind and Point Load Testing.

The mid-span support member is not intended to support point loads from foot traffic and should be of sufficient strength and stiffness to prevent the plastic sheet deflection being greater than that of the adjacent metal sheeting.

Mid-span supports provide the uplift strength required to reduce sheet flutter in high winds. To prevent any movement between adjacent sheets they should be fixed with a type 17 screw and load spreading washer through the GRP and span breaker, and they should be fixed to the metal sheeting.

14.8.6 Purlin Protection

Because of expansion movement, safety mesh will cause damage to the plastic sheeting at the purlin line, and it should be isolated at the purlin. This protection should be a foam tape or other durable and non-absorbent material and be of sufficient thickness and resilience to avoid damage. It should extend the full width of the purlin flange and should include both purlins where purlins are lapped .

When plastic roof lighting is laid over safety mesh or wire netting, protection must be placed over the mesh or netting to avoid damage to the underside of the plastic roof lighting.

14.8.7 Drainage

Acrylic or fibreglass domes should not rely on sealants alone for weathertightness. The design of the penetration flashings should allow for adequate water drainage.

A water diverter-type penetration design must be used for penetrations wider than 600 mm, and proprietary rubber flashings must be placed at 45° to the roof pitch, as described in 9 External Moisture Penetrations.

Acrylic domes must not be used with unpainted galvanised flashings, and plastic roof lighting must not drain onto galvanised roof cladding or gutter.

To avoid inert catchment corrosion galvanised metals must not be used in conjunction with plastic roof lighting.

14.8.8 Condensation

In areas or buildings likely to suffer heavy condensation, the use of a second translucent sheet to form a double skin can be used to minimise condensation. See 11.3 Types Of Roof Lights.

Twin skin plastic sheeting should have a minimum gap of 20 mm and should be spaced with insulating material between the sheets at the fixing points. It is standard practice to place the second sheet above the first after it has been fixed; however, a special ridge is required as shown on Sprung Roof.

The top sheet fasteners penetrate the structure deep enough to resist the negative wind load.

Elevated roof lights, designed specifically for long run tray roofs from ridge to eaves, are fixed to the ribs of the sheet and permit condensation to run off onto the trough.

A UV stabilized grade of clear polythene film or sheeting can also be used in order to help control any condensation dripping from the plastic roof lighting, but its durability may be less than that of the plastic roof sheeting.

14.8.9 Pitch

The minimum pitch is the roof cladding pitch and not the building design roof pitch. The allowable cumulative deflections of the frame, purlin, and roof sheeting require the building design roof pitch to be increased to comply with minimum cladding pitch.

Plastic roof lighting profiles deflect more than metal roof or wall claddings of the same dimension. See 3.5.3 Deflection.

The COP recommends that the roof pitch for plastic roof lighting profiles is increased to more than the minimum for the profile, in addition to that recommended for allowable deflections (1.5°).

It is better practice that all plastic roof lighting profiles with sheets longer than 15 m should be a high rib profile (more than 35 mm) or that the roof pitch should be increased by 3° over that required in 7.1.1 Minimum Roof Cladding Pitch.